It is known that hydrophobic resin sheet and web materials of low conductivity readily become electrostatically charged by friction with dielectric materials and/or contact with electrostatically chargeable transport means, e.g. rollers. The charging occurs particularly easily in a relatively dry atmospheric environment.
Sheets and webs of hydrophobic resins, e.g. polyesters or cellulosetriacetate, are commonly used as support element of recording materials. Such materials are subjected to frictional contact with other elements during their manufacture, e.g. during coating or cutting, and during use, e.g. during the recording of information, e.g. with a step-and-repeat camera or in the case of silver halide photographic materials for X-ray diagnosis during use in filmchangers or when used in film loading and unloading in so-called daylight systems.
Especially in the reeling-up or unreeling of dry photographic film in a camera or projector high friction may occur, resulting in electrostatic charges that may attract dust or cause sparking. In unprocessed photographic silver halide emulsion materials sparking gives rise to developable fog and degrades the image quality.
In order to reduce electrostatic charging of photographic sheet or web materials comprising a hydrophobic resin support coated with at least one silver halide emulsion layer without impairing their transparency it is known to incorporate ionic compounds in these materials, e.g. in the gelatin-silver halide emulsion layer(s) or other hydrophilic colloid layers or in a subbing layer.
In order to avoid diffusion of ionic compounds out of said layers during the wet processing treatments of said materials, preference has been given to incorporate therein antistatic high molecular weight polymeric compounds having ionic groups, e.g. carboxylic sodium salt groups, at frequent intervals in the polymer chain [ref. Photographic Emulsion Chemistry, by G. F. Duffin,--The Focal Press--London and New York (1966)--Focal Press Ltd., p. 168]. To further enhance the permanence of the conductivity of ionic conductive polymers it has been proposed to cross-link these polymers with hydrophobic polymers (ref. e.g. U.S. Pat. Nos. 4,585,730, 4,701,403, 4,589, 570, 5,045,441, EP-A-391 402 and EP-A-420 226).
The conductivity however of an antistatic layer containing said ionic conductive polymers, even after cross-linking, is moisture dependent and is lowered considerably by treatment with an acidic photographic processing liquid, e.g. an acidic photographic fixing liquid or stop bath.
Relatively recently electrically-conducting conjugated polymers have been developed that have electronic conductivity. Representatives of such polymers are described in the periodical Materials & Design Vol. 11, No. 3--June 1990, p. 142-152, and in the book "Science and Applications of Conducting Polymers"--Papers from the 6th European Physical Society Industrial Workshop held in Lothus, Norway, 28-31 May 1990, Edited by W R Salaneck Linkoping University, D T Clark ICI Wilton Materials Research Centre, and E J Samuelson University of Trondheim, published under the Adam Hilger imprint by IOP Publishing Ltd Techno House, Redcliffe Way, Bristol BS16NX, England.
Substances having electronic conductivity instead of ionic conductivity have a conductivity independent from moisture. They are particularly suited for use in the production of antistatic layers with permanent and reproducible conductivity.
Many of the known electronically conductive polymers are highly coloured which makes them less suited for use in photographic materials, but some of them of the group of the polyarenemethylidenes, e.g. polythiophenes and polyisothianaphthene are not prohibitively coloured and transparent, at least when coated in thin layers.
The production of conductive polythiophenes is described in preparation literature mentioned in the above mentioned book: "Science and Applications of Conducting Polymers", p. 92.
The production of colour neutral conducting polymers from isothionaphthene is described in J Electrochem Soc 134, (1987) 46.
For ecological reasons the coating of antistatic layers should proceed where possible from aqueous solutions by using as few as possible organic solvents. The production of antistatic coatings from aqueous coating compositions being dispersions of polythiophenes in the presence of polyanions is described in published European patent application 0 440 957 and corresponding U.S. Ser. No. 647,093 which should be read in conjunction herewith.
It is known that the electrostatic chargeability of polyester resin is high and that for many applications it would be interesting to lower its surface resistance e.g. by applying thereto a conductive primer or subbing layer. It is however also known that it is difficult to establish a good bonding between a polyester resin support and a hydrophilic antistatic layer. In most cases more than one layer is needed to impart sufficient adherence of a hydrohilic colloid layer to a polyester resin support as is the case e.g. in photographic materials, having one or more hydrophilic colloid recording layers such as gelatin-silver halide emulsion layers. Normally, a first special adhesion layer is coated on the support to adhere thereto a proper antistatic layer, which may be protected with a protective layer for avoiding mechanical damage and attack by solvents.
It is common practice to give a polyester film support a sufficient dimensional stability by biaxially stretching and to heat-set it at relatively high temperature. Usually the biaxially orientation of the polyester film support is performed in two stages. First the film is stretched in one direction and afterwards in a direction perpendicular to the first. From an economic point of view it would be advantageous if an antistatic primer layer on the polyester film could be applied either before or between the stretching operations. Applied in the stretching stage the primer layer should retain a good anchorage and its elastic modulus should be such that it easily follows the film enlargement in the stretching. The elastic modulus is the ratio of stress (force per unit area) to strain, the latter being a pure number representing the percentage of elongation (ref. Sears & Zemansky "University Physics", 4th ed.--Addison-Wesley Publishing Company--Reading, Mass., USA, p. 154-155).
After the biaxial stretching the film is conducted through a heat-setting zone wherein the primed polyester film such as a polyethylene terephthalate film is heated until a temperature between 180.degree. and 220.degree. C. is reached, while the film is kept under tension in both directions.
The primer layer should withstand these temperatures without. prohibitive colouration and not loose its conductivity when incorporating electrically conductive material.